Jareth's class takes place at 9:00 A.M. and lasts for three hours straight.
Which means students have to sit and withstand his overwhelming gaze for three hours straight; you can't even move to grab your water bottle and drink water from it; otherwise, you will have to face his cold stares.
It's not that Jareth stops them from drinking water, it's just that his eyes unconsciously fell on the person who makes the most movement in the whole classroom.
Imagine a scenario where the whole class is sitting upright with serious faces and is listening to his words, and you suddenly start moving to drink water… obviously, you will be noticed by him.
And Jareth's gaze is not something everyone wants to attract.
It feels like he is evaluating your patheticness with those arrogant eyes, so it's better to just drink water and do other things before or after his class.
Usually, most student think of his class as stressful but useful.